The aim of this research is to determine the effects of deprivation on visual development by studying children treated for dense, central cataracts. A large sample of these children is being treated at The Hospital for Sick Children by surgically removing the cataract and fitting the resulting aphakie eye with a contact lens. Our objectives are to determine the influence on the visual outcome of: (a) the timing of deprivation, by comparing children treated for congenital, developmental, and traumatic cataracts; (b) the duration of deprivation by comparing children who were first treated at various times after diagnosis of this type of deprivation, by comparing children treated for unilateral versus bilateral cataracts; (c) the extent to which the normal eye was patched (in unilateral cases); and (d) the presence of the additional common problems of persistent hyperplastic primary vitreous microcornea, microphthalmus, nystagmus, secondary glaucoma, and/or strabismus. Because the animal literature suggests that the answers may vary for different visual functions, seven aspects of vision will be measured: (a) grating acuity, (b) better acuity, (c) contrast sensitivity, (d) vernier acuity, (e) peripheral vision, (f) the symmetry of optokinetic nystagmus, and (g) color vision. Standard clinical tests, modified for aphakic eyes, will be used to assess letter acuity, peripheral vision, and color vision. Grating acuity will be measured with preferential looking and optokinetic nystagmus. For contrast sensitivity, an oscilloscope and the method of limits will be used to determine the minimum contrast at which stripes of different spatial frequency are visible. For vernier acuity, a computer-controlled interactive video display and the method of limits will be used to determine the minimum detectable offset between parallel lines. For the symmetry of optokinetic nystagmus (OKN) a large display of moving stripes will be used to determine whether OKN can be elicited easily both when striped move from the temporal field toward the nasal field and when they move in the opposite direction. The results should elucidate the effects of deprivation on visual development, and indicate the optimal treatment for children afflicted with cataracts. The methods refined for this study also will be useful for evaluating the consequences of other visual defects and their treatment, and for screening children for possible eye disorders.